I ran into this problem riding up one of the cable car hills in San Francisco in the middle of a block. A tourist stepped out between 2 parked cars and I had to stop as there was a car to the left of me. Heading back down wasn't an option as there were cable cars, car & pedestrians all around me.

After a couple of attempts to clip in, I pushed off with my right foot, got my left foot moving and managed to crawl ahead. That was fun. Nothing like a dead stop on a 17% grade to test you.